Hydrogen deuteride is a diatomic molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

 composed of the two isotopes of hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

: the majority isotope ¹H protium and ²H deuterium

Deuterium

Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

. Its molecular formula is HD.

Availability in nature

Hydrogen deuteride is a minor component of naturally occurring molecular hydrogen. However, Hydrogen deuteride does not chemically behave exactly the same has H2—and can thus serve as an important marker.

In particular Hydrogen deuteride is one of the minor but noticeable components of the atmospheres of all the giant planets, with abundances from about 30 ppm to about 200 ppm. HD has also been found in Supernova remnants, and other sources.

Gas Giants occourance (HD vs H2)

• Jupiter : ~0.003% :: 89.8% ±2.0%
• Uranus : ~0.007% :: 83.0% ±3.0%
• Neptune : ~0.019% :: 80.0% ±3.2%

Radio emission spectra

HD and H2 do have very similar emission spectra, but don't emit on exactly the same frequencies http://prola.aps.org/abstract/PR/v112/i6/p1929_1.

The frequency of the astronomically important J = 1-0 rotational transition of HD at 2.7 THz has been measured with tunable FIR radiation with an accuracy of 150 kHz http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1988ApJ...330L.135E&db_key=AST.